The fundamental aim of this part of the research program is to analyse the means by which plasma proteins cross the gastric mucosa from the mucosal capillaries to the lumen of the stomach in normal states and in conditions in which the gastric mucosa is injured by substances which cause shedding of plasma proteins. Specifically, the object is to study the movement of plasma from the capillaries into the interstitial space. A segment of the greater curvature of the stomach of an anesthetized dog is placed in a plastic chamber. The segment is supplied with arterial blood through a single cannulated artery, and blood in infused at a constant rate by means of a pump. Perfusion pressure is measured, and vascular resistance is calculated. Effluent venous blood drains from a single cannulated vein and, except when radioactively compounds are injected, blood is returned to the animal which supplies the arterial blood. A bolus injection of plasma labeled with two radioactively labeled plasma proteins in injected intraarterially, and the effluent blood is collected in 58 2-second samples. Comparison of the fraction of injected radioactivity collected in each sample allows calculation of vascular permeability to the proteins. Permeability is measured in the norml state, after intraarterial infusion of histamine, after application of damaging compounds to the mucosa and after administration of histamine-receptor blocking agents. As a concurrent study, factors affecting vascular resistance of the tissue are being studied.